Ultra high frequency generating apparatus



' 1967v I. KAUFMAN ETAL ULTRA HIGH FREQUENCY GENERATING APPARATUS FiledMarch 12, 1963 FREQUENCY INPUT 45 O I O 0 2w OUTPUT GUIDE FREQUENCY w \NPUT EN ERGY INVENTORS //?v//ve KAUFMAN ALLAN 5. R/SLEY United StatesPatent C 3,296,519 ULTRA HIGH FREQUENCY GENERATING APPARATUS IrvingKaufman, Woodland Hills, Calif., and Allan S.

Risley, Boulder, Colo., assignors, by mesne assignments, to TRW Inc., acorporation of Ohio Filed Mar. 12, 1963, Ser. No. 264,543 4 Claims. (Cl.32169) The present invention relates to apparatus for generating ultrahigh frequency energy, and more particularly to a microwave cavitydevice for frequency multiplying.

It is a primary object of the present invention to provide an improvedapparatus for producing microwave energy by means of harmonicgeneration. It is a further object of our invention to provide animproved apparatus for producing microwave energy by means of frequencymixing.

It is a further object of our invention to provide a microwave cavitydevice adapted to simultaneously satisfy the conditions that the deviceis resonant to both an input fundamental frequency signal, to an outputharmonic signal and the condition that a ferromagnetic material elementcontained within the cavity is ferromagnetically resonant at thefrequency of said harmonic signal.

It is a further object of the present invention to provide an improveddoubly resonant microwave cavity device which permits efficient energyconversion and also uses larger ferromagnetic material elements than hasheretofore been possible. It is an additional and related object of ourinvention to provide an ultra high frequency harmonic generator ofsubstantially improved energy conversion efficiency.

A more specific object of our invention is to provide a microwave cavitydevice of the type described which includes means for tuning the cavityto fundamental frequency resonance without altering the figure of meritof the cavity or the resonance of the cavity to a harmonic signal whichis to be generated.

In accordance with a preferred embodiment of our invention, we achievethe foregoing objects and purposes by providing a microwave cavitystructure which allows eificient coupling of power to the frequencyconverting element at either a fundamental frequency or two frequenciesto be mixed and which is resonant at a harmonic frequency, or the sum ordifference frequency respectively. In a particular embodiment, amicrowave cavity structure is resonant in one mode at a fundamentalfrequency and is also resonant in a different mode at a harmonicfrequency. The cavity comprises first and second cavity portions withthe first portion being dimensioned so that it is, by itself, resonantat the harmonic frequency. This first cavity section has disposedtherein a ferromagnetic material element, preferably in the form of athin disc, supporting on one wall of the cavity which element isoperative to generate magnetic fiux at the harmonic frequency whensubjected to both a substantially constant or DC. magnetic flux and afundamental frequency alternating magnetic flux. The first and second.cavity portions are coupled together in a manner to permit propagationof fundamental frequency energy from the second cavity portion into thefirst cavity portion and in a manner to substantially exclude harmonicfrequency magnetic fields from the second cavity portion. Morespecifically, the first and second cavity portions taken together arearranged and dimensioned so that, jointly, they form a single cavitywhich is resonant at the fundamental frequency.

The foregoing and other objects and features of this invention will beapparent from the following description taken with the accompanyingdrawings, throughout Ice which like reference characters indicate likeparts, which drawing forms a part of this application and in which:

FIGURE 1 is a graphical illustration of certain operationalcharacteristics of the invention useful in explaining the advantages ofsame;

FIGURE 2 is a partially broken away perspective view of a cavitystructure in accordance with our invention;

FIGURE 3 is a perspective view of a complete system in accordance'withour invention and embodying a cavity structure similar to that of FIGURE2; and

FIGURE 4 is a cross-sectional view taken along the lines 44 of FIGURE 3.

It has been shown in the prior art that when a microwave field isimpressed on an insulative ferromagnetic material in a manner such thatthe microwave magnetic field has a component normal to the direction ofa DC. magnetizing field, an asymmetric precession of the magnetizationof the material can be achieved and will produce harmonics or secondorder components of magnetic flux. This harmonic frequency magnetic fluxcan be utilized to generate and produce microwave output signals havingfrequencies which are integral multiples of the fundamental inputmicrowave energy. The prior art treatment of the theory of frequencymultiplication by means of ferromagnetic materials will be found, forexample, in (1) an article by J. H. Melchor, W. P. Ayres, and P. H.Vartanian, Proc. Inst. Radio Engrs. 45, 644 (1957), and (2) in anarticle by I. Kaufman, A. S. Risley, and D. D. Douthett, Bull. Am. Phys.Soc. 5, 297 (1960). In the prior art, the foregoing frequencymultiplying effect has been largely a laboratory phenomenon and has notbeen utilized to produce practical magnitudes of microwave power withenergy conversion efliciencies high enough to enable commercialexploitation of the phenomenon. That is, ferrite frequency multiplierswhich have been experimentally operated heretofore have been of littlepractical use for the reason that they have required'rnany kilowatts ofinput power in order to achieve high efficiency. At lower power levels,the efliciencies achieved have been extremely low. For example, one suchfrequency multiplier previously reported in the literature has requiredwatts of 9000 megacycle input power to produce 0.1 watt of output energyat 18,000 megacycles.

The prior art treatment of the theory of frequency mixing'by means offerromagnetic materials will be found, for example, in a report by R. L.Iepsen, entitled Harmonic Generation and Frequency Mixing inFerromagnetic Insulators, Contract No. AF 19(604)1084, HarvardUniversity, May 25, 1958.

In contrast with previous ferrite harmonic generation devices, we havefound that apparatus in accordance with the present inventionincorporating specifically selected materials and dimensional parameterswithin a certain critical range is capable of producing as much as about18 watts of harmonic output energy in response to fundamental frequencyinput power levels of about 300 watts. For the special case of frequencydoubling with ferromagnetic materials in a circuit simultaneouslyresonant to frequencies and 2,, the second harmonic output power P isThe foregoing Equation 1 and its derivation is found in our previouspublication entitled Microwave Harmonic Generation by FerrimagneticCrystals, Journal of Api the ferrite element, and the microwavecircuitry (through the symbol R Equation 1 states that in harmonicgeneration by a volume of nonlinear material (as a ferrite, forexample), the output power is a function of:

( 1) Input power P,,,

(2) Figure of merit of the nonlinear material, F

(3) The coupling impedance R The choice of material is governed by P,,,and F and is independent of the cavity structure. The coupling impedanceis determined by the cavity structure. With the cavity structure of ourpresent invention it is possible and practical to obtain a conversionefliciency of 5% with input power at the 300-watt level, as contrastedto prior devices which have required power levels of the order of onekilo watt to achieve that efiiciency.

To fully understand the criteria for selection of the cavity parameters,we refer to the definition of R To minimize R in accordance with (1), weshould dimension the cavity so that the ratio is a minimum. Now, ingeneral, U is proportional to the product V h where V is the effectivecavity volume at 2w. Therefore R is proportional to This means that acavity should have the minimum volume that allows resonance, while yetmaintaining a high Q. From a practical point of view, the cavitycontaining the ferrite must allow introduction of fundamental frequencypower, must be resonant at and must permit tuning of the w-circuitwithout detuning the 2w-circuit. This is accomplished in the cavitywhich we disclose herein.

By using a cavity structure having two distinct portions and 27 as shownin FIGURE 2, the 2m fields are confined to the lower cavity portion 25so that V is minimized. Since the 2w fields in the portion 27 of thecavity are negligible, the latter may be moved without altering theresonance of the 2w portion 25 of the cavity. Accordingly, therequirements mentioned above have been satisfied. For fundamental tosecond harmonic energy conversion efilciencies smaller than five percentthe figure of merit F of the ferromagnetic material element is given bythe following expression:

In Equation 2, mks units with B H+M are used. M is the saturationmagnetization, e is the ratio of the two transverse components of themagnetization and is minimized by using a thin disc-like ferrite elementpositioned so that 11,, and H are in its plane, and which has Here 17,,is the absorbed power per unit volume; H and h,, are the intensitiesrespectively of the dc and the fundamental frequency magnetic fields. Asstated heretofore, we define is the loaded Q of the 2w circuit and the Uis the energy stored therein at field intensity k Equation 1 assumesthat the ferrite element is uniformly excited, that the 2m circuit ismatched to its load, and that operation is at ferromagnetic resonance atfrequency to. That is, Equation 1 is valid so long as the foregoingthree conditions are substantially satisfied.

Combining Equations 1 and 2, we get an expression for the energyconversion efficiency, 1;

4 Equation 3 can be solved for the value of X required for particularvalues of 7; and P,,. Moreover, since for a ferrite element having aspecified volume V, these chosen values of 7 and P also specify h by thedefinition of X (above). The relation between h,, and X," at variousvalues of input power P is given by the straight line curves 13 and 15of FIGURE 1, for two values of the conversion efiiciency 1;, a specificvalue of R of 4.77 10 ohm-m. and a specific ferrite element having adiameter of 0.20 inch and a thickness of 0.10 inch.

To determine the actual efficiency attainable with a given material, weplot its X vs. h, characteristic, or, rather, the inverse relation h,,vs. X on the graph of FIGURE 1. In general, a particular efficiency canbe achieved with a given material if the line corresponding to thatefficiency intersects or is tangent to the h,, vs. X curve of thatmaterial. For example, if the h, vs. X characteristic of the givenmaterial intersects the curve 13, an etficiency' of 5% can be had withthat material.

The three curves of FIGURE 1 correspond to three different materials.More specifically, the curve 17 of FIGURE 1 is a plot of 11,, vs. X,"for general ceramics type R-l polycrystalline ferrite. Curve 19 is aplot of the same variables for single crystal yttrium-iron-garnet andcurve 23 is a graph of the same relationship for single crystalmanganese ferrite. The straight line curves 13 and 15 of FIGURE 1indicate the relationship between h and X for various fields offundamental frequency input power and for conversion efficiencies of0.5% (curve 15) and 5.0% (curve 13). From examination of the curves ofFIGURE 1, we have selected manganese ferrite as being the materialhaving optimum efliciency of energy conversion from the fundamentalfrequency of 8.5 gc. to the second harmonic of 17.0 gc.

In FIGURE 2, the-re is shown a microwave cavity structure in accordancewith one embodiment of the present invention which is operative inaccordance with the theoretical considerations discussed above.Specifically, the cavity structure of FIGURE 2 comprises a first cavitysection 25 in the form of a generally rectangular structure having sidewalls 29 and 31, a front wall 33, a corresponding rear wall 45, agenerally planar bottom wall 39 and a top wall comprising portions 35and 37 to which are joined a second cavity portion 27. A cylindricalwafer 30 of ferromagnetic material, preferably manganese ferrite, ismounted on the bottom wall 39 substantially at the center thereof. Theferromagnetic element 30 is subjected to a substantially constant or DC.magnetic field provided by any oneof a number of conventionalmagnetizing devices (not shown). The means for applying a DC.magnetization to the element 30 is represented schematically in FIGURE 2by the arrow 32 which indicates that the static magnetization h isgenerally in the direction of the y-axis and in the plane of theferromagnetic material element 30. For excitation of asymmetricprecession in the ferromagnetic element, it is additionally subjected toan alternating magnetic field intensity at the frequency w of the inputmicrowave energy. This magnetic field intensity is applied to the wafer30 substantially in the plane of the wafer and in a direction normal tothat of the DC. magnetization. That is, the cavity structure ispreferably arranged so that the fundamental frequency magnetic flux isapplied to the wafer 30 in the direction of the x-axis. In accordancewith the preferred embodiment the ferromagnetic material element 30 isabout 0.2 in diameter and about 0.01" thick.

The first cavity portion 25 is constructed and dimensioned so that it isresonant at the harmonic frequency which is desired to be generated.Specifically, in this embodiment, the first cavity portion 25 isresonant at 17.0 gc. The 17.0 gc. harmonic field pattern in the firstcavity portion 25 is in the TE mode so that the harmonic frequencymagnetic flux 11 is in the direction of the yaxis and therefore,generally in the plane of the ferromagnetic material wafer 30. Thatsecond harmonic magnetic flux field is indicated by the line 36 inFIGURE 2. The electric field of the second harmonic energy is, ofcourse, normal to the magnetic field and thus the electric field Bextends within the cavity generally normal to the front and rear walls33 and 45.

Fundamental frequency input energy is applied to the cavity structure ofFIGURE 2 from a source of microwave energy which propagates energydownwardly through the second cavity portion 27. The means for applyingthe input fundamental frequency microwave energy is represented by thearrow 41 in FIGURE 2 and will be described in further detail hereinafterin connection With FIGURE 3. The second cavity portion 27 is generallyrectangular comprising side walls 47 and 49, a front wall portion 43 anda rear wall 45. In a preferred embodiment, the dimensions of the secondcavity portion are generally the same as a standard X-band waveguide, sothat the second cavity portion 27 functions as means for propagating 8.5gc. microwave energy into the first cavity portion 25. Because the walls47 and 49 of the second cavity portion are relatively closely spaced,the second harmonic energy which exists in the first cavity portion 25is prohibited from propagating upwardly along the second cavity portion27. That is, the restricting dimensions of the aperture between thefirst and second cavity portions serves to confine the second harmonicfields to the first cavity portion 25 while simultaneously permittingfundamental frequency energy to be fed thereto from the second cavityportion 27.

In FIGURE 3, there is shown a complete frequency multiplying apparatusin accordance with the present invention which includes a doublyresonant cavity structure as described heretobefore in connection withFIGURE 2. Specifically, the first cavity portion 25 is shown at the lefthand end of the apparatus of FIGURE 3 and the second cavity portion 27extends to the right therefrom. At its right hand end, the second cavityportion 27 is provided with a conventional waveguide tuning means whichwon prises essentially a tuning plunger within the end of the waveguide.The tuning plunger is operated by means of a thumb screw 52, to positionit optimumly within the waveguide for tuning the entire system,including the first cavity portion 25 and the second cavity portion 27,to the frequency of the fundamental input energy. That fundamentalfrequency energy is applied to the system by means of a crossedwaveguide coupling element 53 positioned generally perpendicular to thewaveguide cavity portion 27 and contiguously adjacent thereto. Couplingof energy from the crossed Waveguide 53 to the waveguide portion 27 iscontrolled by means of a coupling probe 55 the details of which are moreclearly shown in FIG- URE 4 and described hereinafter. Second harmonicenergy is coupled out of the first cavity portion 25 through a smallaperture in the hidden side wall thereof. An output coupling waveguide50 is connected to that side wall and is utilized to propagate secondharmonic energy outwardly from the system to any desired microwaveutilization means as represented schematically by the arrow 56. In theembodiment shown in FIGURE 3, the output waveguide 50 is returnedgenerally parallel to the waveguide portion 27 in order to facilitatemounting of the cavity structure 25 between the poles of a magnet asindicated schematically by the arrow 32.

In the harmonic generator of our invention, as shown in FIGURES 2 and 3,the fundamental frequency fields fill the entire volume of the cavityportions 25 and 27, while the second harmonic fields are confinedsubstantially to the first cavity portion 25. By isolating the 2wharmonic fields in this manner, we have kept R small by minimizing U andWe have permitted tuning of the to circuit without affecting theresonance of the first cavity portion to the 210 energy. In oneapparatus which we have constructed in accordance with our invention,the 2w cavity 6 was matched to its load and had a Q of 3800. About 90%of the power entering the input guide was absorbed in the ferriteelement 30. In this particular embodiment, there was no need for tuningthe first cavity portion 25, since the fundamental frequency source wastunable, for adjustment of the input frequency and therefore thegenerated harmonic to the natural resonant frequency of the cavityportion 25. It will be appreciated that when a fixed frequencyfundamental signal source is to be used, it will generally be necessaryto provide means for tuning the first cavity portion 25 to the generatedharmonic signal. In such a case, conventional tuning screws, not shown,placed generally parallel to the direction of the 2w electric field inthe cavity portion 25 may be utilized.

The apparatus Was driven with a magnetron (2,12 sec., 40 p.p.s., 8.5gc.). A number of different elements 30 of single crystal MnFe O weretested. In the most efiicient case, the cavity input power was 334 wattsand the output 18.4 watts. The other sample elements 30 were somewhatpoorer but still of about the same order. From FIGURE 1, we could expecthigher eificiencies at higher input powers.

The detailed structure of the crossed waveguide input energy feedingmeans 53 is best illustrated in FIGURE 4 which is a cross-sectional viewtaken in the plane which includes the longitudinal axis of the secondcavity section 27 and the axis of the movable probe 55. Specifically,the feeding means comprises a dielectric cylinder 57 which extendsthrough the walls of the crossed waveguide 53 and inwardly through oneside wall 47 of the Waveguide cavity portion 27. The dielectric cylindercarries at its inner end a metallic coupling stub 59 which absorbsfundamental frequency energy from within the crossed waveguide 53 andradiates that energy to the interior of the second cavity portion 27.The dielectric cylinder 57 is movable axially by means of thumb screw 61to controllably vary the amount of input power which is coupled into thewaveguide cavity portion 27 Thus the crossed waveguide 53 and themovable probe 55 serve to control the amount of fundamental frequencypower which is coupled into the first cavity portion 25 for excitationof the ferromagnetic material wafer 30.

In accordance with a further embodiment of our present invention, wecontemplate that a cavity structure of the type shown in FIGURE 2 may beutilized for heterodyne conversion to obtain the sum or differencefrequency of two separate input microwave signals. That is, two separateinput signals may be applied to the wave guide portion 27 by means offirst and second coupling devices similar to the crossed waveguide 53 asshown in FIGURE 3. These two separate input signals are propagatedsimultaneously along the waveguide portion 27 to the first cavityportion 25 and jointly excite the ferromagnetic element 30. By virtue ofthe nonlinear characteristics of the element 30, a heterodyne mixing ofthe two input signals may be accomplished so that either the sum or thedifference frequency may be extracted from the first cavity portion 25by means of an output circuit generally'similar to the output waveguide50 of the apparatus of FIG- URE 3.

While the present invention has been shown in one form only, it will beobvious to those skilled in the art that it is not so limited but issusceptible to various changes and modifications without departing fromthe spirit and scope thereof.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A frequency-multiplying microwave cavity device comprising incombination:

a cavity resonant in one mode at a fundamental frequency and resonant ina different mode at a harmonic frequency, said cavity having a generallyT- shaped cross-sectional configuration with the crossbar of the T beinga first cavity portion and the leg of the T being a second cavityportion having a longitudinal axis intersecting the crossbar back wall,said first cavity portion being dimensioned to be alone resonant at theharmonic frequency and said second cavity portion being dimensioned topropagate fundamental frequency energy into said first cavity portionbut to substantially exclude harmonic frequency energy generated in saidfirst cavity portion from said second cavity portion to provide maximumcoupling of fundamental frequency energy to said first cavity portion,

a ferromagnetic material element disposed entirely within said firstcavity portion on said longitudinal axis and crossbar back wall, saidelement being capable of generating magnetic flux at the harmonicfrequency when subjected to a DC. magnetic flux in the plane of saidferromagnetic material element and a fundamental frequency magnetic fluxsubstantially in the plane of said element in a direction normal to thatof the DC. magnetic flux,

energy input means for exciting said cavity to resonate at thefundamental frequency so that said ferromagnetic material element iscaused to produce magnetic flux at the harmonic frequency in thedirection of the DC. magnetic flux and in the plane of saidferromagnetic material element,

and tuning means associated with the outer end of said leg for tuningsaid cavity to said fundamental frequency without altering the harmonicfrequency resonance of said first cavity portion.

References Cited by the Examiner UNITED STATES PATENTS 3,041,524 6/1962Karayianis et a1 321-69 3,044,021 7/1962 Poole et a1. 330-4.8 3,054,0429/1962 Weiss 321-69 3,076,941 3/1963 Yariv 330-4.9 3,229,193 1/1966 T.Schaug-Petterson 321-69 FOREIGN PATENTS 833,130 4/1960 Great Britain.

OTHER REFERENCES Microwave Frequency Doubling From 9 to 18 KMC inFerrites by Melchor, Ayres and Vartanian; Proceedings of IRE (1957);pages 643646.

JOHN F. COUCH, Primary Examiner.

G. GOLDBERG, Assistant Examiner.

1. A FREQUENCY-MULTIPLYING MICROWAVE CAVITY DEVICE COMPRISING INCOMBINATION: A CAVITY RESONANT IN ONE MODE AT A FUNDAMENTAL FREQUENCYAND RESONANT IN A DIFFERENT MODE AT A HARMONIC FREQUENCY, SAID CAVITYHAVING A GENERALLY TSHAPED CROSS-SECTIONAL CONFIGURATION WITH THECROSSBAR OF THE T BEING A FIRST CAVITY PORTION AND THE LEG OF THE TBEING A SECOND CAVITY PORTION HAVING A LONGITUDINAL AXIS INTERSECTINGTHE CROSSBAR BACK WALL, SAID FIRST CAVITY PORTION BEING DIMENSIONED TOBE ALONE RESONANT AT THE HARMONIC FREQUENCY AND SAID SECOND CAVITYPORTION BEING DIMENSIONED TO PROPAGATE FUNDAMENTAL FREQUENCY ENERGY INTOSAID FIRST CAVITY PORTION BUT TO SUBSTANTIALLY EXCLUDE HARMONICFREQUENCY ENERGY GENERATED IN SAID FIRST CAVITY PORTION FROM SAID SECONDCAVITY PORTION TO PROVIDE MAXIMUM COUPLING OF FUNDAMENTAL FREQUENCYENERGY TO SAID FIRST CAVITY PORTION, A FERROMAGNETIC MATERIAL ELEMENTDISPOSED ENTIRELY WITHIN SAID FIRST CAVITY PORTION ON SAID LONGITUDINALAXIS AND CROSSBAR BACK WALL, SAID ELEMENT BEING CAPABLE OF GENERATINGMAGNETIC FLUX AT THE HARMONIC FREQUENCY WHEN SUBJECTED TO A D.C.MAGNETIC FLUX IN THE PLANE OF SAID FERROMAGNETIC MATERIAL ELEMENT AND AFUNDAMENTAL FREQUENCY MAGNETIC FLUX SUBSTANTIALLY IN THE PLANE OF SAIDELEMENT IN A DIRECTION NORMAL TO THAT OF THE D.C. MAGNETIC FLUX, ENERGYINPUT MEANS FOR EXCITING SAID CAVITY TO RESONATE AT THE FUNDAMENTALFREQUENCY SO THAT SAID FERROMAGNETIC MATERIAL ELEMENT IS CAUSED TOPRODUCE MAGNETIC FLUX AT THE HARMONIC FREQUENCY IN THE DIRECTION OF THED.C. MAGNETIC FLUX AND IN THE PLANE OF SAID FERROMAGNETIC MATERIALELEMENT, AND TUNING MEANS ASSOCIATED WITH THE OUTER END OF SAID LEG FORTUNING SAID CAVITY TO SAID FUNDAMENTAL FREQUENCY WITHOUT ALTERING THEHARMONIC FREQUENCY RESONANCE OF SAID FIRST CAVITY PORTION.